Spade bits (sometimes also known as "fly cutters") are widely used for boring holes between approximately 1/4" and 11/2" diameter in wood and similar soft materials, because they are fast, true cutting, resharpenable, and relatively inexpensive. Their name derives from the shape of the spadelike blade or cutter, in contrast to the spiral shape of augers and twist drills. The blade, which is usually forged integrally from the shank, is relatively thin and flat and may have a width several times the diameter of the shank.
Robinson U.S. Pat. No. 2,782,824, issued Feb. 26, 1957, to the assignee of the present application, shows a bit of this general type, having a flat blade with straight outer sides extending parallel to the axis, shoulders which extend radially inwardly from the outer sides, and a sharp triangular central point which projects from the shoulders along the direction of the axis. The point centers and steadies the blade with respect to the workpiece and prevents it from wandering as it cuts; the shoulders cut the hole to the diameter of the outer sides; and the outer sides serve to stabilize or center the blade when it cuts through the workpiece. The outer sides, shoulders, and point are all slightly beveled along the edges to provide a "positive rake" (i.e., a slightly acute angle, typically about 86.degree., with the blade surface) for cutting. On each cutting or leading edge of the point (the two diagonally opposite edges in the direction of rotation) a groove or flute is formed which extends back to and somewhat beyond the shoulder, onto the flat face of the blade. These point flutes extend parallel to the cutting edges of the point; one flute is formed on each surface of the point. These flutes are nearly parallel to the axis and assist in the upward removal of chips cut by the point. This enables the point to penetrate and cut the wood with less force and thrust than would otherwise be required.
In use, spade bits of relatively small diameters, approximately 1/2" or less, penetrate relatively quickly, for a given loading but for blade diameters above approximately 3/4", the rate of cutting is markedly slower. Moreover, as the blade emerges from the opposite (or back) side of the workpiece, having cut through it, the centering function of the point is progressively lost and the blade is then guided only by its outer sides. As the blade shoulders emerge on the opposite face of the workpiece, splitting and tearing tend to occur, with increased likelihood of jamming and damage to the tool and the workpiece.
It is known to form a forwardly projecting sharp cutting foot on auger bits, see for example, Kronwall U.S. Pat. No. 2,627,292. However, such forwardly projecting cutting edges require an increase in the thickness of the blade at the cutting edge, which in turn greatly increases cost of auger bits relative to that of a spade-type bit wherein the blade is forged to a uniform thickness.